1. Field of the Invention
The present invention relates to an image encryption method and device for partially encrypting images
2. Prior Art
Encryption/decryption conversions are accomplished by means of an encryption algorithm which is controlled by an encryption key. Encryption techniques are described in detail in “Ango to Joho Security (“Cryptographs and Information Security)” written by Tsujii and Kasahara and published in March, 1990 by Shokodo, a publishing company.
A typical cryptograph is described in U.S. Pat. No. 3,958,081 issued on May 18, 1976. This cryptograph is called DES (see “Data Encryption Standard”, FIPS IMAGE ENCRYPTION METHOD AND DEVICE PUB, NBS January, 1977). In this DES (data encryption standard) cipher, the encryption is done by 64-bit unit.
The above-described encryption is called a symmetrical block cryptograph; and in contrast to this, there exist asymmetrical public-key cryptographs. Typical RSA (Rivest, Shamir, and Adleman) cipher is described in “A Method for Obtaining Digital Signatures and Public-Key Cryptosystems”, a publication called “Communications of the ACM”, Vol. 21, No. 2, pp. 120-126, (1978) as a public-key cryptograph.
These techniques are superior, and the present invention is based on these techniques. However, the present invention is not limited to the use of these techniques alone; and various encryption techniques including these techniques can be used in the present invention.
Furthermore, in Japanese Patent Application Laid-Open (Kokai) No. S63-212276, titled “Facsimile Device”, a facsimile device is described. In this facsimile device, important portions of the image on the original paper to be transmitted are encrypted, while the remaining portions of the image are transmitted “as it is”. In this system, a portion of the image data on the original paper is designated by a marker and is encrypted at the time of data transmission. Then, partially encrypted dots are outputted in the original plain-text position on the outputted paper on the receiver side. However, there is no detailed description or disclosure of the encryption processing on the transmission side in this prior art. Furthermore, since the dot density is ordinarily 8 dots/mm, if the portion where the encryption is initiated is shifted by even one dot in this prior art, the output that resembles noise is obtained. Accordingly, considering the fact that data is read while being scanned in the horizontal direction using paper as the input-output medium, the reading of handwritten designated regions and encrypted portions is extremely difficult; and such a technique is difficult to realize using an ordinary facsimile machine, and therefore the invention may be imperfect. Of course, information transmission techniques which thus use paper as an information medium belong to a different technical field than information transmission techniques for transmitting electronic image data, which is the object of the present invention. For example, in the case of color image data, one dot is expressed by 24 bits combining colors of R, G and B (red, green and blue).
Furthermore, U.S. Pat. No. 5,504,818, “Information Processing System Using Error-Correcting Cryptographs and Cryptography” and Japanese Patent Application No. H3-215909 “Intellectual Information Processing Method and Device” are known prior art, which disclose inventions created by the inventor of the present application. In these prior arts, portions of images or drawings are extracted as rectangles, these portions are filled by blanks, the extracted portions are encrypted, and the encrypted portions are stored in items linked to the original items. In this method, linked files are necessary for the encrypted data.